The POSIX function putenv() is used to set environment variable values. The putenv() function does not create a copy of the string supplied to it as an argument; rather, it inserts a pointer to the string into the environment array. If a pointer to a buffer of automatic storage duration is supplied as an argument to putenv(), the memory allocated for that buffer may be overwritten when the containing function returns and stack memory is recycled. This behavior is noted in the Open Group Base Specifications, Issue 6 [Open Group 2004]:

A potential error is to call putenv() with an automatic variable as the argument, then return from the calling function while string is still part of the environment.

The actual problem occurs when passing a pointer to an automatic variable to putenv(). An automatic pointer to a static buffer would work as intended.

Noncompliant Code Example

In this noncompliant code example, a pointer to a buffer of automatic storage duration is used as an argument to putenv() [Dowd 2006]. The TEST environment variable may take on an unintended value if it is accessed after func() has returned and the stack frame containing env has been recycled.

Note that this example also violates DCL30-C. Declare objects with appropriate storage durations.

int func(const char *var) {
  char env[1024];
  int retval = snprintf(env, sizeof(env),"TEST=%s", var);
  if (retval < 0 || (size_t)retval >= sizeof(env)) {
    /* Handle error */
  }

  return putenv(env);
}

Compliant Solution (static)

This compliant solution uses a static array for the argument to putenv().

int func(const char *var) {
  static char env[1024];

  int retval = snprintf(env, sizeof(env),"TEST=%s", var);
  if (retval < 0 || (size_t)retval >= sizeof(env)) {
    /* Handle error */
  }

  return putenv(env);
}

According to the [Open Group 2004] entry for putenv():

...the string pointed to by string shall become part of the environment, so altering the string shall change the environment.

This means that the call to putenv() is only necessary the first time func() is called, since subsequent changes to the string update the environment. If func() were called more than once, an additional variable could be added to avoid calling it unnecessarily.

Compliant Solution (Heap Memory)

This compliant solution dynamically allocates memory for the argument to putenv():

int func(const char *var) {
  const char *env_format = "TEST=%s";
  const size_t len = strlen(var) + strlen(env_format);
  char *env = (char *) malloc(len);
  if (env == NULL) {
    return -1;
  }
  int retval = snprintf(env, len, env_format, var);
  if (retval < 0 || (size_t)retval >= len) {
    /* Handle error */
  }
  if (putenv(env) != 0) {
    free(env);
    return -1;
  }
  return 0;
}

The POSIX setenv() function is preferred over this function [Open Group 2004]. In particular, using putenv() will necessarily leak memory if called multiple times for the same environment variable, due to restrictions on when you can safely free the old value. According to the [Open Group 2004] entry for putenv():

Although the space used by string is no longer used once a new string which defines name is passed to putenv(), if any thread in the application has used getenv() to retrieve a pointer to this variable, it should not be freed by calling free(). If the changed environment variable is one known by the system (such as the locale environment variables) the application should never free the buffer used by earlier calls to putenv() for the same variable.

Compliant Solution (setenv())

The setenv() function allocates heap memory for environment variables, which eliminates the possibility of accessing volatile stack memory:

int func(const char *var) {
  return setenv("TEST", var, 1);
}

Using setenv() is easier and consequently less error prone than using putenv().

Risk Assessment

Providing a pointer to a buffer of automatic storage duration as an argument to putenv() may cause that buffer to take on an unintended value. Depending on how and when the buffer is used, it can cause unexpected program behavior or possibly allow an attacker to run arbitrary code.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported: Can be checked with appropriate analysis stubs.
Axivion Bauhaus Suite		CertC-POS34
CodeSonar		(customization) BADFUNC.PUTENV	Users can add a custom check for all uses of putenv(). Use of putenv
Compass/ROSE
Helix QAC		C5024
Klocwork		CERT.PUTENV.AUTO_VARIABLE
Parasoft C/C++test		CERT_C-POS34-a CERT_C-POS34-b	Usage of system properties (environment variables) should be restricted Do not call putenv() with a pointer to an automatic variable as the argument
PC-lint Plus		2601	Fully supported
Polyspace Bug Finder		CERT C: Rule POS34-C	Checks for use of automatic variable as putenv-family function argument (rule fully covered)

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-252/CWE-253/CWE-391 and ERR33-C/POS34-C

Independent( ERR33-C, POS54-C, FLP32-C, ERR34-C)

Intersection( CWE-252, CWE-253) = Ø

CWE-391 = Union( CWE-252, CWE-253)

CWE-391 = Union( ERR33-C, POS34-C, list) where list =

• Ignoring return values of functions outside the C or POSIX standard libraries

Bibliography